This application aims at improving our understanding of the frequency and characteristics of nave B cells that are the precursors of those that produce VRC01-class broadly neutralizing antibodies (bNAbs). VRC01-class bNAbs bind the CD4-binding site of the HIV Env and are among the most potent and broadly neutralizing antibodies known. They protect animals from experimental infection and when administered passively to chronically-infected patients they reduce plasma viremia. VRC01-like antibodies are therefore the type of antibodies one would want to elicit by vaccination. So far, however, this has not been achieved. We do know quite a few things about the mutated forms of VRC01-class antibodies, but there are many critical gaps in our knowledge of the germline form of these antibodies. Lab-engineered 'predicted' germline forms of such antibodies do not recognize Env. This information has led us and others to hypothesize that recombinant Envs fail to stimulate nave B cells expressing germline VRC01-class B cell receptors (BCRs). However, the true nature of germline VRC01-class antibodies/BCRs is not known, so many of the assumptions we make about their infrequent elicitation during infection and their non-elicitation during immunization may be inaccurate. Here we will employ innovative and complementary methodologies and an interactive experimental approach to define the frequencies of nave human B cells expressing germline VRC01-class BCRs, to define the true germline amino acid sequences of these antibodies/BCRs and to define their Env-recognition properties. The information gathered by our proposed studies will be critical to better understand how HIV-1 interacts with particular nave B cells and also will be important for the design of Env-based immunogens capable of engaging the broadest possible array of germline VRC01-class BCRs.